R-1. Bagchi, P., "Process for the Precipitation of Stable Colloidal Dispersions of Base Degradable Components of Photographic Systems in the Absence of Polymeric Steric Stabilizers," U.S. Pat. No. 4,933,270. PA1 R-2. Bagchi, P., "Methods of Preparation of Precipitated Coupler Dispersions With Increased Photographic Activity," U.S. Pat. No. 4.970,139. PA1 R-3. Bagchi, P., Beck, J. T., and Crede, L. A., "Methods of Forming Stable Dispersions of Photographic Materials," U.S. Pat. No. 4,990,431. PA1 R-4. Bagchi, P., Sargeant, S. J., Beck, J. T., and Thomas, B., "Polymer Co-precipitated Coupler Dispersions," U.S. Pat. No. 5,091,296. PA1 R-5. Bagchi, P. and Sargeant, S. J., "Increased Photographic Activity Precipitated Coupler Dispersions Prepared by Coprecipitation With Liquid Carboxylic Acids," U.S. Pat. No. 5,104,776. PA1 R-6. Bagchi, P., McSweeney, and Sargeant, S. J., "Preparation of Low Viscosity Small-Particle Photographic Dispersions in Gelatin," U.S. Pat. No. 5,013,640. PA1 R-7. Bagchi, P., Edwards, J. L., Gibson, D., Rosiek, T. A., Thomas, B., and Flow, V. J., "High Dye Stability, High Activity, Low Stain and Low Viscosity Small-Particle Yellow Dispersion Melt for Color Paper and Other Photographic Systems," U.S. application Ser. No. 627,154. PA1 R-8. Ono, Y., Yoneyama, H., and Ueda, H., "Dispersions Containing Surface Active Agents With Units of Polyoxyethylene and Polyoxypropylene," U.S. Pat. No. 3,860,425. PA1 R-9. Kruyt, H. R., "Colloid Science," Vol. I & Vol. II, Elsevier, Amsterdam (1952). PA1 R-10. Anonymous, "Photographic Silver Halide Emulsions, Preparations, Addenda Processing and Systems," Research Disclosure, 308, p. 933-1015 (1989). PA1 R-11. Chen, B., "Laser Light Scattering," Academic Press, N.Y., 1974. PA1 R-12. Barker, T. B., "Quality By Experimental Design," Dekker, N.Y., (1985). PA1 R-13. Anonymous, "SAS User's Guide; Statistics," Version 5 Edition, SAS Institute, North Carolina (1985).
It has been known in the photographic arts to precipitate photographic materials, such as couplers, from solvent solution. The precipitation of such materials can generally be accomplished by a shift in the content of a water miscible solvent (R-1) and/or a shift in pH (R-2 to R-7). The precipitation by a shift in the content of water miscible solvent is normally accomplished by the addition of an excess of water to a solvent solution. The excess of water, in which the photographic component is insoluble, will cause precipitation of the photographic component as small particles. In precipitation by pH shift, a photographic component is dissolved in a solvent that is either acidic or basic. The pH is then shifted such that acidic solutions are made basic or basic solutions are made acidic in order to precipitate particles of the photographic component which is insoluble at that pH. Such precipitation techniques, in the absence of a latex polymer, lead to microprecipitated dispersions (R-1 to R-3 and R-5 to R-7). Such microprecipitated dispersions have been termed as "microprecipitated slurries" (MPS), as at this stage no gelatin has been added to the dispersion. The microprecipitated dispersions have relatively narrow particle size distribution compared to conventional milled dispersion prepared by milling in the presence of gelatin as described by Ono et al (R-8). Polymer co-precipitated (PCP) dispersions can be precipitated by similar pH-shift mechanism in the presence of a base-ionizing group combining polymer latex where, after precipitation, the photographic agent gets loaded inside the polymer latex particles (R-4). In PCP dispersions, the particle size is of the order of the polymer particles which can be anywhere between 50 to 800 nm.
Microprecipitated dispersions of the types mentioned above are generally prepared in the absence of gelatin. For the purpose of coating, it is necessary to add gelatin to such dispersions. It has been found earlier that small particle microprecipitated dispersions, when admixed with gelatin, produce excessive melt viscosities that are unsuitable for preparation of photographic coatings, single layer, or multilayer (R-6). There are two probable explanations for high viscosity of gelatin melts of such small-particle melts. The first cause is possibly due to the relatively higher increase in the excluded volume of the small-particle melts compared to conventional large-particle dispersions due to the presence of the gelatin adsorption layer as indicated in (R-6) column 3, line 38, as appended by reference. The second possible explanation lies in the much higher surface area of the small-particle dispersions. Conventional milled dispersions have relatively broad size distributions, and their mean diameters lie between 100 and 1000 nm, preferably between 100 and 400 nm. For the purpose of this invention, we define such conventional milled dispersions as "large-particle dispersions". MPS or PCP dispersions are usually much smaller in size and have very narrow size distribution. For the purpose of this invention, we define such dispersions with particle diameter smaller than 100 nm as "small-particle dispersions".
The specific surface area S (surface area per unit weight) of a dispersion system is given by: EQU S=6/.rho.D (I)
where .rho. is the density of the particle and D is the mean diameter of the particles, assuming narrow size distribution. FIG. 1 illustrates the dependence of the specific surface area (with the approximation that .rho.=1.0 g/cc) and the weight of gelatin needed to saturate the particle surface [assuming saturation gelatin adsorption is about 10 mg/m.sup.2 as indicated in (R-6)], in the range of sizes covering both small- and large-particle dispersions. It is seen in FIG. 1 that for large-particle milled dispersions, saturation gelatin need is about 1 g gelatin per g of the dispersed medium. However, that for the small-particle dispersions, depending upon size, the saturation gelatin need is between 1.0 and 100 g of gelatin per g of the dispersed material. In a coating melt the ratio of gelatin to dispersed phase is between about 0.5 to about 2.0. Use of larger amounts of gelatin than the conventional range leads to thicker coating layers and, hence, loss of sharpness in the photographic product. Therefore, use of normal gelatin levels in small-particle dispersions leads to fractional surface coverage and, hence, "bridging" of dispersed particles (et. FIG. 2) which results in high viscosity melts. In older literature, bridging of particles have been described as "sensitized flocculation" (R-9).
The high viscosity problem has been solved by the use of certain viscosity-control surfactants to the gelatin solution before addition of the small-particle microprecipitated dispersion very rapidly (R-6 and R-7). It is hypothesized that the viscosity-control surfactants attach themselves to the hydrophobic segments of the gelatin molecule and the particle surface, and thus prevents or retards strong attachment of the gelatin molecule to particles by steric hindrance and thus essentially eliminate "sensitized flocculation". It has been pointed out in the prior art reference (R-7) that the mixing of the dispersion and the gelatin has to be fast in order to avoid sensitized flocculation. Fast mixing can be easily achived in small laboratory scale preparation of gelled melts. In production scale, where very large volumes of dispersion and surfactant gelatin solutions require mixing, normal mixing by addition of one solution to another can not be achieved very fast. It will be shown in the examples that when such addition and mixing is carried out, the turbidity of the resulting dispersion depends upon the rate of addition of gelatin. This leads to undesirable variability in production of the quality of the dispersion formed.
Microprecipitated dispersions have many advantages over conventional milled dispersions. Many solvent-free microprecipitated dispersions of photographic agents can provide dispersions that are much more active than their conventional milled analogs as described in references (R-3), (R-6), and (R-7). Other microprecipitated dispersions can be rendered active by incorporation of a polymer latex (R-6), high boiling coupler solvents (R-2), or liquid carboxylic acids (R-5). Many microprecipitated dispersions of photographic couplers produce dyes that are much more stable to fade compound to their conventional analogs (R-3), (R-6), and (R-7).